Electrochemical oxidation of phenol



United States Patent 3,509,031 ELECTROCHEMICAL OXIDATION OF PHENOL Frank H. Covitz, Lebanon, N.J., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Continuation-impart of application Ser. No.

616,453, Feb. 16, 1967. This application Aug. 28, 1968, Ser. No. 755,821

Int. Cl. B01k 1/00; C07c 49/62, 49/74 US. Cl. 204-78 18 Claims ABSTRACT OF THE DISCLOSURE The electrochemical oxidation of phenol to hydroquinone or benzoquinone was achieved at high chemical and electrical efiiciencies by electrolyzing aqueous solutions of phenol at temperatures of 25 to about 100 C., a pH of less than about 4 at a current density of 4 to about 100 amperes/dm. until up to about 80% by weight of the phenol had been electrolyzed This is a continuation-in-part of Ser. No. 616,453, filed Feb. 16, 1967 which in turn is a continuation-in-part of Ser. No. 593,309 filed Nov. 10, 1966, both now abandoned.

This invention relates to the electrochemical oxidation of phenols and more particularly to the preparation of hydroquinone in aqueous solution.

Although the electrochemical oxidation of phenol has been described in the chemical literature, the use of this technique as a practical preparation of hydroquinone has never been realized because of the plethora of reaction products and the low efficiencies of the electrochemical oxidations carried out. For example, several papers disclose that large amounts of ortho benzoquinone and catechol are formed as contaminating by-products in the electrochemical oxidation of phenol. The prior art discloses electrical efficiencies of no greater than about 30 percent and chemical efficiencies (moles of hydroquinone obtained per mole of phenol consumed) of no greater than 50%. The state of the prior art is not surprising in view of the fact that many variables are involved in any electrochemical reaction and in particular in elec trochemical oxidations. It is particularly difficult to control all of these variables to achieve a single effect such as the formation of only one product at a high electrical and chemical efiiciency. Examples of the variables which must be considered include: cell configuration, choice of electrode material, electrode potential, current density, temperature, electrolyte composition, phenol concentration, time of reaction, percent conversion, and the like as well as the effects of these variables on one another.

It has now been found that a method for preparing hydroquinone at chemical efficiencies of 90-95% with a minimum of by-products has been achieved at overall electrical efiiciencies of about 60% by a method which comprises the steps of:

(a) Electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and about 1 to 35 percent by weight of an electrolyte at a temperature of 25 to about 100 C. and a pH of less than about 4 between an anode having a DC. potential of at least about +0.9 volt in reference to a saturated calomel electrode and a cathode having a cathode potential more ice negative than about +.04 volt in reference to a saturated calomel electrode and at a current density of at least 4 amperes per square decimeter until up to about percent by weight of the phenol has been electrolyzed to hydroquinone; and

(b) Recovering the hydroquinone from the aqueous solution.

It has also been discovered that if the electrolysis of phenol is carried out in a divided cell, that is one in which the anode and cathode are separated by means of a semipermeable or ionically permeable membrane such that the anolyte contacts only the anode and the catholyte touches the cathode, then the phenol is converted to p-benzoquinone rather than to hydroquinone. This product p-benzoquinone is the only measurable product formed in the anolyte. Cellophane membranes can be used.

The present invention can be practiced either as a batch or a continuous process. The electrical efiiciency of the undivided electrolytic cell, that is one not having a semipermeable membrane interposed between anode and cathode, is about 10 percent lower than that of the divided cell having the membrane, because at high conversions the hydroquinone formed at the cathode can be reoxidized at the anode to para-benzoquinone. Such a cyclic conversion consumes electricity without generating more product and therefore lowers the apparent electrical efliciency. The undivided cell however is preferred from the standpoint of simpler design, construction and maintenance. Electrical efiiciency in either case is always lower than chemical efiiciency due to water electrolysis.

The features of the present invention which distinguish it from the prior art and which indeed surprisingly provide a method for obtaining hydroquinone at high chemical and overall electrical efiiciencies with a minimum of by-product formation are a combination of several variables. First of all the concentration of phenol in the aqueous solution being electrolyzed is limited to an upper figure of about 4 percent by weight, and a lower practical limit of about 0.5 percent by weight; Although the concentration of phenol in the aqueous solution can be as high as about 4 percent by weight, a higher chemical efiiciency is obtained at 3 percent by weight or less. This chemical efliciency is of the order of about percent at low phenol concentrations. Above 4 percent phenol conconcentration, conversion efliciency to para-benzoquinone drops to about 50 percent or lower, the remainder appearing as tars or low molecular weight condensation products which adhere to the electrode surface. Tar formation as mentioned before is responsible for the most part in reducing electrical efiiciency as the electrolysis proceeds. This is the general effect obtained when one practices the methods described in the prior art. It is preferred to use a phenol concentration range of about 1-3 percent phenol with a range of 2-3 percent being particularly preferred. In this range it is preferred that no more than 50% by weight of phenol be electrolyzed in any one batch or pass in the case of a continuous process.

When this invention is practiced as a batch process, the upper temperature limit was found to be about 60 C. Above this temperature quinone becomes moderately unstable to the electrolysis procedure accompanied by considerable evaporation of the aqueous solution. The lower limit is 25 C. because below this point quinhydrone begins to crystallize out of solution. At the temperature ranges used in the practice of this invention the hydro quinone electrolysis product remains dissolved in the aqueous phase. For the preparation of hydroquinone, the temperature of 25-40 is preferred with 30-40 being particularly preferred. Both conductivity and diifusion rates are considerably increased as the temperature is raised from room temperature.

When this invention is practiced as a continuous process, the upper temperature limit is not critical because the residence time is much shorter than in a batch process thereby minimizing thermal side reactions. While temperatures of about 25 to 100 C. can be used, it is preferred to operate continuously in the range of about 30 to 80 C., with the range of about 40-70 C. being more preferred.

The concentration of electrolyte in the electrolysis bath is not narrowly critical, but it is preferred to operate in the range of about 1 to percent by weight when the electrolyte is an inorganic acid. This range is preferred because of the relatively minor variations in electrical efficiency over this range as well as the obtention of sufficient conductivity to allow high current densities without excessive heating from PR power dissipation in a well designated cell. When the electrolyte consists of a mixture of an inorganic acid and an ionizable salt, said salt concentration can range from about 1 to percent by weight, or 1 to .20 percent by weight and even as high as about 30 percent by weight. Although concentrations below 1 percent may also be used, the reaction is markedly slower.

Suitable electrolytes comprise any materials which ionize readily in water at a pH of 4 or less and preferably 2 or less and do not interfere with the phenol electrolysis. Specific examples include inorganic acids such as sulfuric acid, phosphoric acid and the like; inorganic salts such as sodium sulfate, sodium bisulfate, potassium sulfate, potassium bisulfate, lithium sulfate, lithium bisulfate, sodium phosphate and the like in conjunction with sufficient inorganic acid to maintain a pH of 4 or less; and organic salts such as sodium acetate, potassium acetate, lithium acetate, and the like in a solution acidified to a pH of 4 or less.

In order to maintain optimum anode efficiency, at the low end of the current density range specified, one may clean the electrodes by immersion in a hydrocarbon tar solvent, as for example, ketones, such as acetone, alcohols such as ethanol or aromatic hydrocarbons such as benzene, toluene and the like. This can be done by a rinsing operation in which the electrodes are left intact mounted in the electrolytic cell.

In the most preferred ranges of current density later 7 defined, this cleaning operation is not required.

An extremely important variable of the present invention is the anode potential. This value has a minimum below which oxidation of phenol by electrolysis will not occur. This value is approximately +0.9 volt with respect to a saturated calomel electrode as measured by standard voltammetric techniques. It is preferred to employ an anode potential of about +0.9 to +3.5 volts with a range of +1.8 to +3.0 volts being more preferred. While the cathode potential should be less than about +0.4 volt, it is preferred to operate below about l.0 volt.

For the same considerations the current density is also important since for a given system the electrode potential is usually a single-valued function of the current density. The electrode potential may therefore be controlled by adjusting the current density. Also since the conversion rate per unit area of electrode surface is determined as much by the current density as by the electrical efficiency, it is desirable to operate at high. current densities. The preferred range of current densities for the oxidation of phenol by electrolysis is about 4 to 100 amperes/dmfi. While the upper limit can be exceeded it is not practical to operate below 4 amperes/dm. because of tar formation on the anode which lowers the efiiciency of the phenol electrolysis to a point where the oxidative process is no longer commercially feasible. The discovery, that carrying out the oxidation at high current densities not only ameliorated the process with respect to lessening of tar formation on the anode but did so Without a concomitant marked drop in electrical efficiency, was completely unexpected and contrary to what one skilled in the art of electrochemistry would expect. The use of high current densities in the practice of this invention also carries with it the added economic advantage of reducing the number of cells required for the electrolytic oxidation of phenol.

A more preferred range of current densities is about 20 to 100 amperes/dm. with about 30 to amperes/ dm. being particularly preferred and about 40 to 60 amperes/dm. most preferred.

As to the choice of anode materials, any metal which is stable in the electrolyte or which passivates under the electrolysis condition may be used, such as for example, platinum, gold, graphite, manganese, chromium, iron, nickel, lead and alloys thereof and the like. Lead in the presence of an electrolyte containing sulfate ion is prefered as the anode material for oxidation of phenol to p-benzoquinone. In this system insoluble lead sulfate is formed on the lead anode which is further oxidized to lead dioxide. The oxidation of phenol is then actually effected by a lead dioxide anode. As the cathode material, any metal which is stable to the electrolysis conditions described in the present invention and which has a high hydrogen over-voltage may be employed as the cathode in the present invention. Suitable cathode materials include such metals as mercury, lead, tin, cadmium, copper, nickel and the like. Lead is preferred as the cathode material for the reduction of p-benzoquinone to hydroquinone. Of course it is to be understood that the list of these materials mentioned as being useful as anodes and cathodes is not exclusive and any of the metals which meet the above qualifications may be used if desired.

The configuration of the electrolytic cell used in the present invention is not narrowly critical. The only important point to be considered is whether the cell constitutes a divided or undivided cell, that is, whether the anode is physically separated from the cathode so that one has separate anolytes and catholytes connected by means of a semi-permeable membrane. As pointed out before, a divided cell provides as the main product of the electrolysis of phenol, p-benzoquinone, whereas the undivided cell affords hydroquinone.

The degree of conversion to which the phenol substrate is subjected is critical. For example, after about 50 percent conversion of phenol to hydroquinone by electrolysis, the overall electrical efliciency begins to drop. Approximately 80 percent was therefore determined as the upper practical limit of conversion of phenol. The formation of side products becomes more appreciable above the 80 percent phenol conversion level and aside from the depreciation in yield, conversion beyond this point is undesirable because of tar sticking to the electrode surface which results in reduction in the rate of diffusion of phenol to the anode.

Pressure is not at all critical and so while it is preferred to use atmospheric pressure for convenience, superatmospheric as well as subatmospheric pressures may be employed if desired.

The isolation and purification of the electrolysis prodnets of phenol can be effected by several techniques. In the case of the undivided electrolytic cell as mentioned above, the product is almost entirely hydroquinone. Thus during the course of the electrolysis, the electrolysis solution will consist of dissolved hydroquinone, unreacted phenol, water, electrolyte, and traces of quinone. By allowing the concentration of hydroquinone to increase sufficiently in the electrolytic bath, removal of this prod- 'well at the low concentration levels encountered in the electrolysis cell involves continuous extraction techniques. The product stream can be continuously extracted with a low boiling, water-immiscible solvent, such as diethyl ether, in which the organic materials are soluble. The ether stream can be then fed into a vessel containing a hi her boiling solvent in which both phenol and p-benzoquinone are soluble but in which hydroquinone is quite insoluble, such as, carbon tetrachloride. The ether is recycled and the hydroquinone filtered from the carbon tetrachloride solution and subsequently recrystallized in conventional apparatus. The carbon tetrachloride filtrate which contains traces of p-benzoquinone, as well as unreacted phenol, can be re-extracted with the aqueous phase from the first extraction and reused. The aqueous phase can then be returned to the electrolysis cell after appropriate replenishment with phenol.

In the case of a divided electrolytic cell, if the contents of the anode chamber are serially circulated into the cathode chamber thereby reducing the electrolytically formed quinone to hydroquinone, the isolation procedures described above for the undivided cell can be used.

The divided cell however can be used to isolate and purify p-benzoquinone itself as a reaction product or as a purified intermediate for further conversion to hydroquinone. Thus for example, the contents of the anode chamber can be crystallized at low temperatures to obtain a phenol-quinone complex in pure form. Separation by fractional distillation can then be used to afford pure p-benzoquinone and recover unreacted phenol. Alternatively the electrolysis mixture can be extracted with an organic solvent, and after evaporation of the solvent the p-benzoquinone can be isolated by fractional distillation.

The invention is further described by the examples which follow in which all parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 The electrolysis cell was fabricated from a 1.5 liter beaker having an 11 x 7 centimeter lead anode centrally mounted in the beaker connected through a tab of lead at the top to a heavy gauge copper wire and an alligator clip. Two electrically common cathodes fabricated from 3 cm. strips of lead encased in cellophane tubing (as a membrane) were mounted symmetrically in the beaker facing the anode one on either side of the anode at a ,distance approximately 3 cm. One liter of an aqueous solution containing 3 percent by weight of phenol and 3 percent by weight of sulfuric acid was placed in the beaker with the anode positioned below the surface of the solution. The solution was agitated during the electrolysis by a magnetic stirrer rotating at the bottom of the beaker below the electrodes. A constant current of 18 amperes (current density of 12 amperes per dm?) was passed between the electrodes for one hour. During this time the temperature of the solution was maintained by means of a water cooling bath at 30 C. The anode potential measured with respect to a saturated calomel electrode positioned close to the anode was 2.101.0 2 volts. The concentration of p-benzoquinone was measured polarographically and found to be 0.085 molar indicating a conversion of 26 percent at an overall electrical efficiency of 50 percent. The electric power used in this experiment was supplied by means of a conventional DC. power supply well known in the art.

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the apparatus was modified by making the total effective area of the cathodes equal to that of the anodes and eliminating the use of the cellophane membrane. After 3 hours reaction at 18 amperes, the concentration of hydroquinone was found to be 0.068 molar indicating a conversion of 21 percent at a 40 percent overall electrical efficiency.

EXAMPLE 3 The procedure described in Example 2 was repeated with the exception that 120 amperes (current density of amperes per dm?) were passed for 0.5 hour through the electrolysis cell. The temperature of the bath rose to 60 C. during this time. The percent conversion to hydroquinone was measured and found to be 61.2 percent at an electrical efficiency of 35 percent.

EXAMPLE 4 The procedure described in Example 2 was followed with the exception that the phenol concentration was 1 percent and 6 amperes were passed through the cells for 1 hour. The conversion to hydroquinone was 31.6 percent at an overall electrical efficiency of 60 percent.

EXAMPLE 5 The procedure described in Example 4 was followed with the exception that 6 amperes were passed for 4.5 hours. The conversion to hydroquinone was 80 percent at an overall electrical efiiciency of 35.4 percent.

CONTROL A The procedure described in Example 2 was used except that 5 percent phenol concentration was used and 6 amperes were passed through the cell for 5 hours. The conversion to hydroquinone was 12.1 percent at an overall electrical efficiency of 23 percent.

EXAMPLE 6 The procedure used in Example 2 was followed with the exception that a potentiostat was used as the power source instead of the conventional DC. power supply. The anode potential was controlled at +1.8 volts as measured against a standard calomel electrode. After 18 ampere hours had passed through the electrolytic cell, as measured with an electromechanical integrator, the conversion to hydroquinone was measured at 28.3 percent at an overall electrical efiiciency of 54.5 percent.

EXAMPLE 7 The procedure described in Example 2 was used with the exception that 6 amperes were passed through the electrolytic cell for 8 hours. The conversion to hydroquinone was then measured at 6 3 percent at an overall electrical efficiency of 45 percent.

EXAMPLE 8 The procedure described in Example 7 was used with the exception that the anode was cleaned with acetone every hour. After the passage of 6 amperes for 8 hours the conversion to hydroquinone was measured at 70 percent at an overall electrical efficiency of 50 percent.

EXAMPLE 9 An electrolysis cell was fabricated consisting of 6 anodes and 7 cathodes fabricated from sheet lead and positioned such that each provided 1.2 dm. per side in contact with the electrolyte. The electrodes were spaced with Teflon dividers and gasketed to prevent leakage (Teflon is a trademark for polytetrafluoroethylene). The electrolysis cell was provided with inlet and outlet tubes so that electrolyte could be passed through the cell continuously. Cellophane can be optionally inserted between the anodes and cathodes to provide a divided cell. A total of 14.4 dm. available active anode surface was thus provided in a volume of 500 cc. of electrolyte. Ten liters of an aqueous solution containing 3 percent by weight of phenol and 3 percent by weight of sulfuric acid was circulated by means of a peristaltic pump connected to the cell chambers by means of manifold input and outlet connections. The cell was operated in the undivided mode, that is, without the use of membranes inserted between the anodes and cathodes, with the passage of 120 amperes (current density of 8.3 amperes per dm?) over a period of 4 hours at an anode to cathode potential of 3.8 volts. During this time the temperature of the electrolyte rose to 50 C. The conversion to hydroquinone was found to be 56 percent at an overall electrical efficiency of 40 percent. An aqueous solution containing 15 grams of sodium sulfite was added to the electrolyte and after removal from the electrolysis cell, the total aqueous electrolyte solution was continuously extracted with ether for 12 hours. After this time the ether solution was removed from the continuous liquid-liquid extractor, decolorized with 15 grams of activated charcoal, filtered, and the ethereal solution evaporated to less than A of its original value. Carbon tetrachloride was added until the precipitation was complete and the resulting slurry was cooled to C. The slurry was then filtered and the precipitate washed 0n the filter with fresh carbon tetrachloride. This precipitate, hydroquinone, after drying amounted to 157 grams indicating a recovery efiiciency of 95 percent. Recrystallization from dioxane or water afforded 140 grams of white crystals of high purity. The carbon tetrachloride filtrate from above was evaporated leaving behind a residue 117 grams of unreacted phenol, thus indicating that the chemical efficiency in conversion of phenol to hydroquinone in the above-described electrolytic cell was 88 percent.

EXAMPLE A unit cell was fabricated from a 600 ml. water-jacketed beaker with two lead cathodes of approximately /2 dm. each, mounted on opposite sides of the vessel. A lead anode (3 x 8 cm.) with a central tab was suspended in the center of the beaker with its sides parallel to the cathodes. Five-hundred milliliters of an aqueous solution containing 3% by weight phenol and 3% by weight H 80 was introduced into the beaker and the height of the anode adjusted so that the 3 x 8 cm. section was just immersed.

Three such cells were connected in electrical series arrangement and 40 amps (80 amps/dm?) were passed for 0.75 hour. At that time the conversion was 56%, indieating 32% electrical efiiciency. A total of g. of pure hydroquinone was isolated by the techniques described in Example 9.

The electrolysis described above was repeated twentyfive times in succession without cleaning or otherwise disturbing the anodes. At the end of this time, the over-all electrical efficiency was not significantly lower than the value obtained after the first run.

At a current density of 4 amps/dmP, the electrical efficiency dropped to one-half its original value after n ne successive runs without cleaning. At this point, the electrode activity was completely restored by removing tar from the anodes by means of acetone rinses.

The identity of the hydroquinone product obtained in the present invention was demonstrated by polarographic, NMR (nuclear magnetic resonance) and infrared analyses well known in the art. The purity of the hydroquinone produced was better than about 99% as demonstrated by a melting point (uncorrected for stem immersion) of 172 C. and a mixed melting point with an authentic sample of photographic grade, hydroquinone obtained from Eastman Kodak Co. which showed no depression. In addition the hydroquinone produced was sufficiently 8 pure to permit its polymerization to a polyhydroxyether using the method described in US. 2,602,075.

Although the invention has been described with great particularity, it is understood that many changes and modifications can be made without departing from the spirit and scope of the invention.

What is claimed is:

1. Method of preparing hydroquinone which comprises the steps of:

(a) electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and about 1 to 35 percent by weight of an electrolyte at a temperature of about 25 to 100 C., a pH of less than about 4, an anode DC. potential of at least about +0.9 volt in reference to a saturated calomel electrode, a cathode potential more negative than +0.4 volt in reference to a saturated calomel electrode, and a current density of at least 4 amperes per square decimeter until up to about percent by weight of the phenol has been electrolyzed to hydroquinone; and

(b) recovering the hydroquinone from the aqueous solution.

2. Method claimed in claim 1 wherein the anode is fabricated from lead and the electrolyte contains sulfate 10118.

3. Method claimed in claim 1 wherein the anode potential is at least +1.8 volts and the cathode potential is more negative than 1.0 volt.

4. Method claimed in claim 1 wherein the current density is about 20 to amperes per square decimeter.

5. Method claimed in claim 1 wherein the current density is about 40 to 60 amperes per square decimeter.

6. Method claimed in claim 1 wherein the temperature is about 25 to 40 C.

7. Method claimed in claim 1 wherein the electrolyte is an inorganic acid and is present in an amount ranging from about 2 to 5 percent by weight.

8. Method claimed in claim 7 wherein the electrolyte is sulfuric acid.

9. Method claimed in claim 1 wherein the electrolyte is a mixture of about 1 to 5% by weight of an inorganic acid and about 1 to 30% by weight of an ionizable salt.

10. Method claimed in claim 9 wherein the electrolyte is a mixture of sulfuric acid and sodium sulfate.

11. Method claimed in claim 1 wherein the electrolyte is sodium bisulfate.

12. Method claimed in claim 1 wherein the aqueous solution contains from about 1 to 3 percent by weight of phenol and the pH is less than about 2.

13. Method claimed in claim 1 wherein the electrolysis is continued until up to about 50 percent by weight of the phenol has been electrolyzed to hydroquinone.

14. Method claimed in claim 1 wherein a continuous el-ectroylsis process is employed.

15. Method claimed in claim 14 wherein up to about 8-0 percent of the phenol has been electrolyzed to hydroquinone.

16. Method of preparing p-benzoquinone which comprises the steps of:

(a) electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of phenol and l to 5 percent by weight of an electrolyte at a temperature of about 25 to 60 C. and a pH of less than about 4 between an anode having an anode DC. potential of at least about +0.9 volt in reference to a saturated calomel electrode and a cathode physically isolated from the anode by means of an ionically permeable membrane with the DC. potential of said cathode being more negative than about +0.4 volt in reference to a saturated calomel electrode and a current density of at least 10 amperes per square decimeter until up to about 80 percent by weight of the phenol has been electrolyzed to p-benzoquinone, and

(b) recovering the p-benzoquinone from the aqueous solution.

17. Method claimed in claim 16 wherein the ionically permeable membrane is cellophane.

18. Method claimed in claim 16 wherein the electrodes are fabricated from lead and the electrolyte contains sulfate ions.

References Cited 11/1938 Vagenius et al. 204-78 10 10 OTHER REFERENCES Allen, Milton 1., Oxidation of Aromatic Compounds, Organic Electrode Processes, Chapman & Hall Ltd., London, 1958, pp. 125-126.

Shields et al., Transaction of Electrochemical Society, vol. 80, pp. 113119.

HOWARD S. WILLIAMS, Primary Examiner H. M. FLOURNOY, Assistant Examiner 

